1. Field of the Invention
The present invention relates to an apparatus for controlling the electroplating of a strip being passed sequentially through a plurality of plating cells.
2. Description of the Prior Art
Prior art apparatus for controlling electroplating of a continuous strip through a plurality of plating cells has included automatic control circuitry for controlling the total current applied to the plurality of cells as a function of a measured speed of the strip in order to maintain an even plating thickness on the strip when the line speed increases or decreases. A total current value or criterion is initially set based upon one or more factors such as the desired plating thickness, electrode efficiency and the width of the strip together with the minimum line speed of the strip through the plating cells.
FIG. 1 is a block diagram of a conventional plating current control apparatus. A strip 1 is moved in the direction indicated by the arrow successively through plating cells 3a, 3b, 3c and 3d by a conventional drive mechanism (not shown) for electroplating a desired thickness on the strip. Sensors, such as current sensing shunts 4a, 4b, 4c and 4d detect the plating currents passing to the respective plating cells from respective rectifier circuits 5a, 5b, 5c and 5d, which are shown as being silicon controlled power rectifier circuits. Controllers 6a, 6b, 6c and 6d, controlled by feedback signals from the sensors 4a, 4b, 4c and 4d as well as by current demand signals from distributors 7a, 7b, 7c and 7d, operate the rectifier circuits 5a, 5b, 5c and 5d to generate the desired plating currents. An adder 8 sums the current magnitude signals from the detectors 4a, 4b, 4c and 4d and applies the sum to a proportional and integral (PI) controller 9 through an adder 10a. The output of PI controller 9 is connected to inputs of the distributors 7a, 7b, 7c and 7d. The adder 10a has a second input from arithmetic circuit 10 generating a desired total current signal which is the product of a set current signal from current criterion circuit 11 and a speed signal from tachometer 2 mechanically coupled to a wheel engaging the strip 1. The current criterion circuit calculates a total current criterion from a desired amount of plating on the strip 1, the length of the plating cells and the electrode efficiency at a minimum line speed.
In operation of the apparatus of FIG. 1, the arithmetic circuit 10 multiplies the signal from current criterion circuit 11 by the speed signal from tachometer 12 to thus generate a desired total current signal which varies in accordance with the speed of the strip 1 through the plating cells 3a-3d so as to maintain the production of a uniform plating thickness on the strip 1 during variations in the speed of the strip 1. The output of the adder 8, which is the sum of the plating currents sensed by detectors 4a, 4b, 4c and 4d has a sign opposite to that of the output of the arithmetic circuit 10 so that the output of adder 10a is the difference between the total measured current and the desired total current. The PI controller produces an output which is adjusted up or down in accordance with integration of the signal from adder 10a to produce a signal which is adjusted up or down only when the output of adder 8 differs from the output of arithmetic circuit 10. The output of PI controller 9 is distributed equally by distributor circuits 7a, 7b, 7c and 7d to the inputs of controllers 6a, 6b, 6c and 6d which in turn control the rectifier circuits 5a, 5b, 5c and 5d in a conventional manner to produce plating currents in the cells 3a, 3b, 3c and 3d to maintain a uniform plating thickness irrespective of the speed of the strip 1 through the plating cells.
Although the conventional plating control apparatus can maintain a total current through the plating cells which varies in accordance with line speed, there still exists deficiencies in the plating caused by line speed variations, such as a deficiency in the gloss of the plated surface, a variation in plating thickness resulting from a variation in electrode efficiency at different line speeds, and a deficiency in anti-corrosive characteristic of the plating. It is proposed that these deficiencies can be reduced by maintaining a plating current density within a predetermined range. The prior art plating control apparatus cannot maintain a plating current density in the predetermined range while simultaneously controlling the total plating current in accordance with variations in the line speed.